1. Technical Field of the Invention
The present invention relates to an engine torque control apparatus used for controlling torque in an engine control system, and to a method for adjusting the engine torque control apparatus. In particular, the present invention relates to an engine torque control apparatus which can be appropriately applied to an engine control system provided, in particular, with an EGR (exhaust gas recirculation) system, and to a method for adjusting the engine torque control apparatus.
2. Related Art
As is well known, an EGR (exhaust gas recirculation) system generally includes an EGR passage for permitting communication between an intake passage and an exhaust passage of an engine, and an EGR valve for permitting channel areas to be variable in these passages. Generally, in such an engine control system having an EGR system, an ECU (electronic control unit), which is provided in the engine control system to mainly perform engine control, adjusts opening angle of the EGR valve to partially recirculate (return) the exhaust gas passing through the exhaust passage of the engine to the intake passage via the EGR passage. Specifically, the ECU in this engine control system adjusts opening angle of the EGR valve on the basis of successively inputted various sensor signals indicating the engine operation conditions, with reference, for example, to an adjustment map. Thus, an optimum recirculated uncombusted fuel quantity (map adjustment value) is obtained, which quantity is suitable for the engine operation conditions of the time. An explanation will be given below about a general mode of use of an EGR system.
Such an EGR system is generally used in an engine (internal combustion engine), or a diesel engine, in particular, which is used for motive power for an automobile, for example, so as to improve exhaust characteristics. Diesel engines combusting fuel by self-ignition are generally operated at a lean air-fuel ratio (whose fuel proportion is lower than a theoretical air-fuel ratio) during a steady operation. Therefore, the reaction between oxygen (O2) and nitrogen (N2) is accelerated under high-temperature conditions, tending to produce NOx (nitrogen oxides), or environmental pollutants. Thus, the amount of emission of NOx in generally used diesel engines has been larger than that of spark ignition engines (so-called gasoline engines) which are generally operated at a theoretical air-fuel ratio during steady operation. The EGR system mentioned above is used therefore in such a diesel engine to reduce the amount of emission of NOx. In other words, as mentioned above, by providing such an EGR system in a diesel engine, the exhaust gas flowing through an exhaust passage can be partially recirculated (returned) to an intake passage to mitigate the increase of combustion temperature in the engine. Mitigation in the increase of combustion temperature in an engine suppresses NOx from being produced by the combustion. As disclosed in Japanese Patent Publication (Laid-open) No. 6-108824, for example, such an EGR system is loaded on a number of engine control systems, without being limited to diesel engine control systems, to improve the exhaust characteristics of the engines.
Generally, in engine control systems, including the engine control systems provided with the EGR systems mentioned above, an amount of fuel (which generally corresponds to an injection quantity (injection ratio) of a fuel injection valve as a fuel supply device) supplied for combustion in the engine, or spark timing for the fuel, for example, is controlled so as to match a value suitable (suitable value) for the engine operation conditions (including the opening angle of the EGR valve), with the use of a given map (or an alternative mathematical formula or the like). In this way, torque produced by combustion in the engine, and thus shaft torque (output torque) reflecting loss, for example, is controlled to a desired magnitude. In particular, during steady operation of the engine, such a control mode enables torque control with reliably sufficient accuracy. However, the inventors of the present application have found that such a control mode still has room for improvement, in particular, in the map (or mathematical formula or the like) used in the control.
Recently, combustion is very often carried out at a rich air-fuel is ratio in diesel engines loaded, for example, on automobiles. For example, to cope with the recently tightened emission regulation, combustion at a rich air-fuel ratio (whose fuel proportion is higher than a theoretical air-fuel ratio) is often performed in a temporal manner in the engine for the purpose, for example, of regenerating the exhaust purification catalyst or DPF (diesel particulate filter). In this case in particular, the air-fuel ratio is switched from the lean air-fuel ratio of the steady operation to a rich air-fuel ratio, and then again switched to the lean air-fuel ratio. The inventors have conducted various experiments assuming several cases including the above case. As a result, the inventors confirmed: that, in an engine control system having the EGR system, in particular, a positive (plus) fluctuation is caused in the output torque when the air-fuel ratio is switched (changed) to a leaner air-fuel ratio; and that, further, the drivability of the vehicle is deteriorated when the output torque is shifted from a target value (map adjustment value).
The present invention has been made in light of the circumstances described above, and has as its object to provide an engine torque control apparatus and a method for adjusting the engine torque control apparatus, which are able to maintain high drivability when an air-fuel ratio is changed to a leaner air-fuel ratio in an engine control system having the EGR system.